SN55, SN55S, SN55V, SN75, SN75S, SN75V User Manual
2: Description
02250247-571 R04
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2.8
Variable speed drive (VSD)
components
The VSD, located in the machine’s electrical enclosure,
works in concert with the controller to allow the compres-
sor to match its output to the current demand of the sys-
tem. The drive’s heat sink extends through the back of
the enclosure, and its cooled by air flowing through the
compressor enclosure.
2.9
VSD control system, functional
description
Refer to
Figure 2-5
. The controls consist of:
• a VSD
• a solenoid valve
• a regulating valve
• an inlet valve
Depending on the model, a compressor can be operated
at a setpoint pressure from 60 to 175 psig (4.1 to
12.1 bar). The controller automatically sets the frequency
range based on the selected pressure. (The compres-
sor's operating range is on its nameplate.)
The following paragraphs apply to a compressor with a
100 psig (6.9 bar) operating pressure and a 6 psi
(0.4 bar) load delta setting.
Compressors with different pressure operating ranges
perform in the same manner.
Start mode—0 to 50 psig (0 to 3.5 bar)
Pressing the controller start button signals the VSD to
accelerate the motor to full speed, causing the separator/
sump tank pressure to rise from 0 to 50 psig (0 to
3.4 bar). At this time both the pressure regulator and
solenoid valves are closed; the inlet valve is fully open
and the air-end provides a full flow to the separator/sump
tank. A minimum pressure valve set at approximately
50 psig (3.4 bar) isolates the rising compressor air pres-
sure from the service line.
Full load mode—50 to 100 psig (3.4 to 6.9 bar)
When the compressed air pressure rises above 50 psig
(3.4 bar) the minimum pressure valve opens allowing
compressed air to flow into to the service line. From this
point on the controller monitors the line pressure which
controls the VSD. The pressure regulator and solenoid
valves remain closed with the inlet valve fully open, run-
ning at 100 psig (6.9 bar) or lower.
VSD part load control
The service line pressure increases to a value above
100 psig (6.9 bar) if the demand is less than the com-
pressor's rated capacity. In this condition, the VSD slows
the motor's rpm which reduces the output to match the
demand. The drive continuously adjusts the motor's rpm
to maintain a 100 psig (6.9 bar) line pressure.The con-
troller maintains the correct frequency when the VSD is
operating in this mode.
Unload mode—in excess of 106 psig (7.3 bar)
When there is no demand or it is at a minimal level, the
service line pressure continues to rise. When it exceeds
106 psig (7.3 bar), or reaches a preset unload pressure
value, the controller de-energizes the solenoid valve
allowing separator/sump tank air pressure to be supplied
directly which closes the inlet valve. The solenoid valve
simultaneously sends a pneumatic signal to the blow
down valve which opens to the atmosphere, and reduces
the separator/sump tank pressure. The check valve in the
air service line prevents line pressure from back-flowing
to the separator/sump tank. The compressor will shut
down after the programmed unload time setting expires.
When the line pressure drops to the low pressure setting
of 100 psig (6.9 bar) the controller starts the motor and
energizes the solenoid valve which closes the blowdown
valve. The re-energized solenoid valve prevents line
pressure from reaching the inlet control valve, thereby
allowing it to fully open, and the compressor supplies
compressed air to the system.
NOTE
The load delta default setting is 10 psi (0.7 bar).
Sullair recommends a setting of 6 psi (0.4 bar)
for the most efficient operation.
